Role of the solvent in large crystal grain growth of inorganic-organic halide FA0.8Cs0.2PbIxBr3 − x perovskite thin films monitored by ellipsometry

Abstract

The role of an additional solvent thiosemicarbazide (TSC) in the synthesis of a highly crystallized inorganic–organic halide FA0.8Cs0.2PbIxBr3 − x perovskite using the co-solvent N,N-dimethylformamide/1-cyclohexyl-2-pyrrolidone has been investigated. Ellipsometry combined with x-ray diffraction and FTIR spectroscopy was used for characterization purposes. The average crystal grain size increased from several hundred nanometers to 1–1.5 μm with increasing content of TSC in the co-solvent. Ellipsometry and FTIR spectroscopy revealed that the as-deposited perovskite film before thermal annealing was distorted and featured a large number of voids; also, the Pb ions formed complexes with the C˭S groups in TSC, which suppressed the excessive nucleation. Removal of the co-solvent at the thermal annealing stage facilitated the rearrangement of the [Pb(IxBr3 − x)6]4− octahedral structure from a zero-dimensional to a three-dimensional network, which enhanced crystal grain growth.

Hybrid Perovskite Degradation from an Optical Perspective: A Spectroscopic Ellipsometry Study from the Deep Ultraviolet to the Middle Infrared

“…Various peaks corresponding mostly to molecular bond vibrational modes within the FA + and MA + cations can be observed. The assignments of all known peaks [ 6,34–37 ] are detailed in Table 1 . The absorption spectrum is shown in terms of k instead of the absorption coefficient α = 4π k /λ, as is customary, because the volumetric bond density corresponding to each peak is then directly proportional to the peak area, [ 38 ] that is, the area of each sub‐peak shown in red.…”

“…A prominent concern is the rate at which MA and FA are depleted at temperatures of 100 °C and above, with substantial effects already after 30–60 min. Given that most standard solution processing recipes require annealing at similar temperatures and durations [ 1–10,17,20,23,27,34–37,42 ] (including the ones used in this work), concerns regarding the actual stoichiometry of pristine perovskite films can arise. Indeed, the data presented here suggests that, depending on annealing time, it is likely that some of the organic cations will have been lost during fabrication, leading to final film stoichiometries that differ significantly from the perovskite stock solutions.…”

“…This was attributed to a phase instability similar to that observed in FAPbI 3 and CsPbI 3 , [ 27,43,45,46 ] leading to the observed formation of non‐perovskite δ‐CsPbI 3 within films with this composition, in addition to PbI 2 . In all cases, the significant changes in film composition during annealing at temperatures around 100 °C, which are often done during film fabrication, [ 1–10,17,20,23,27,34–37,42 ] raise concerns over the assumed initial stoichiometries of the films.…”

Solution-Processed Monolithic Tandem Perovskite/n-Si Hybrid Solar Cells Using MoO3/InZnO Bilayer-Based Interconnecting and Window Layers

“…Here, 300-350 nm thick FA 0.9 Cs 0.1 PbI 3 films were synthesized by adjusting the mixing ratio of DMF/CHP using the difference in the vapor pressure and boiling temperature between DMF (153 • C, 4 × 10 2 Pa) and CHP (286 • C, 7 Pa) with no use of anti-solvent method. In addition, the thiosemicarbazide (TSC) was added to the starting material to enlarge the crystal grain size, resulting in an average grain size of 1-1.5 µm [31]. Then, 60 wt% TPFB-doped PTAA was spin coated as HTL, which has better thermal stability compared to spiro-OMeTAD and can be stored at room temperature.…”